I recommend using the access plugs on both sides of the cowl (fenders must be removed). Run a pressure washer through it to "eliminate" the elimination. Consider adding some bleach to the mix. Be careful not to shoot directly on the back of the fins so as not to bend them. They are fragile in my experience.
The GTA I'm working on now was similar in that there were no rust holes, but there was a couple coats of paint. I poured aircraft stripper through the vents. Then I took a 3/4" diameter 3 foot long dowel rod and drilled a hole in the end. Bought a couple of 3/4" copper pipe cleaner brushes and cut the wrapped wire off at the handle, sticking it in the end of the dowel. It will fit through the access hole, and by using the sturdy access hole as a fulcrum, you can bear down and get some pressure on the brush end, scrubbing the stripper. Another pressure washer run will help clean it out. I think 3 or 4 applications were enough for me to get it squeaky clean.
On another car I had some surface rust, but no holes. For that I took some 1/4" steel tubing, put a 90" sweep elbow, and pinched the end off to make a poor man's nozzle. Adapted to my pressure blaster, and through the same access hole was able to dust the whole area to clean it up.
The bottom line is to avoid sticking anything through the vents, and don't blast them. They warp if you look at them sideways.
Also don't blast if you don't need to - the steel is galvanized. If you can remove the paint and it is clean, don't blast.
